The present invention relates to an electrical apparatus for differential protection, notably a circuit breaker or switch, comprising a trip device of the kind comprising a differential current detection transformer, a trip relay of a mechanism such as the breaking mechanism of the circuit breaker, and a test circuit for checking the operation of the trip device, said relay comprising a fixed yoke bearing a coil, and a blade or similar mobile with respect to the yoke, the relay and transformer being electrically connected in such a way that in normal operation of the apparatus the blade is held in the position pressing against the fixed yoke, and that when a differential current is detected by the transformer core, the blade actuates the aforementioned breaking mechanism.
Differential trip devices are apparatuses which are designed based on relays having a great sensitivity. Under certain climatic conditions, the mobile element, that is to say the blade, may stick to the fixed element and prevent the differential protection from operating. In addition, these apparatuses, due to their nature, remain permanently closed in order to provide continuity of service and are only operated very seldom. This is why they are often provided with a test circuit which a checker can operate locally or remotely in order to check the correct operation of the protection.
Electrical apparatuses for differential protection of the kind referred to above are known, as described in the documents FR-2,638,909 or EP-264,313, in which the test circuit is arranged as a branched leakage circuit connected between two connection points of the active conductors of the power system, respectively line-side and load-side of the transformer and mainly comprising a pair of test resistors, a test button of the push-button type and a protection switch. Manual closing of the test button simulates a differential current by insertion of the resistors in the test circuit, which current is detected by the transformer which gives rise to differential tripping by the relay, causing automatic opening of the breaking device contacts. Tripping of the relay in addition causes automatic opening of the protection switch to disconnect the test circuit. This type of apparatus can also comprise a remote tripping circuit, closing of which can simulate a remote isolation fault.
However, each time the test circuit is operated, whether locally or remotely, tripping of the apparatus takes place and the electrical continuity is therefore interrupted. This electrical continuity, which has to be re-established, requires the intervention of an operator whose job it is to reclose the apparatus.
The present invention solves these problems and proposes a differential protection apparatus with a test circuit of simple design, making the checker's job easier while at the same time improving the reliability of the differential protection.